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Posted on rec.autos.sport.rally
Mike Hall <michael.hall@lineone.net> wrote in message
> Does anybody here know what makes this sound, heard under braking, on
WorldRallyCars? Is it transmission, turbo, or braking?

REWORDED QUESTION:  Why do world rally cars and other turbo race cars, and
the S1 Quattro and the vintage IMSA/Transam turbo Audis CHIRP like crazy and
almost all street turbo setups don't CHIRP at all????  This question has
driven me nuts so i resolved to get an answer.

===following is a composite of my thoughts and two knowledgable audi guys
and may be more than you wanted to know about an arcane subject ;-) ====

Many/most all turbo cars on the street have a bypass valve. (blowoff or dump
valves are maybe a different term, ie to atmosphere direct instead of into
intake system)

My '92 Audi S4 and many other turbo audis and porsches I run with at track
events have high boost, bypass valves and NOT A SINGLE CHIRP TO BE HEARD.

So, what's the technical difference?  One theory is that CHIRPY cars like
the world rally cars dump their pressure into the intake plumbing upstream
of the cold side compressor  where street cars' bypassed pressure goes, but
street cars intake systems are designed to very effectively dampen the
chirping and whooshing noises. Whereas race cars have no sound damping and
very direct paths to atmosphere. The other theory is that it is a blowoff
valve that dumps directly into atmosphere and not into the intake system.

These intake noise damping techniques must be effective, because on my '92
S4 which develops 26psi of boost and 330hp at the track (with race fuel)
uses a stock bypass valve with several inches of hose back into the intake

Here's the bypass valve theory of operation from a manual for the famous
inline-five, 20-valve Audi street motor, which has the same basic design as
the world-champion Quattro rally cars of the late-80's:

"The bypass valve is used to reduce boost pressure in the intake air duct
when the throttle plate is closed, such as, at idle, or when decelerating.
The use of the bypass valve serves another function which is to maintain a
higher speed of the turbo with the throttle closed. This improves the
response of the turbo when going back to acceleration. The bypass valve is
operated by intake manifold vacuum, During idle or deceleration with the
throttle valve closed, the bypass valve is opened by vacuum against spring
pressure. When the valve is open, intake air is recirculated and the turbo
is free to spin, but doesn't develop boost pressure in the intake system.
This maintains a higher speed of the turbo and improves spin-up time.

<you are exactly right, its the dump valve going to the atmosphere, through
the intake system. you would be amazed, but the intake track is almost as
noisy as the exhaust. if you were to remove your airbox you would hear a
whooshing much more pronounced than you do now.  the auto manufacturers
spent a lot of
engineering time trying to get rid of the intake noise.>

<sometimes I can hear a slight chirp when I'm running 27psi with my Greddy
high volume blowoff valve, but usually the exhaust drowns it out.>

if you go to an import drag event you will hear a lot of chirps from the
turbo Hondas using a speed density air metering system.  with it,  you can
dump before the throttle because it takes its measurements in the intake

the Audi 200 Trans-Am has a wide open intake horn that is connected directly
to the turbo inlet. Kind of like a speaker megaphone. The A2 Quattros from
the group B era also could be setup this way with a different air intake
horn. Without a bypass valve, or even with one and extremely high boost
levels, the boost charge will blow back through the turbo impeller  (cold
side) during shifting and this is what causes the chirp chirp chirp sound.

<<But I don't think the air reversing out the airhorn would make that sound
because it's a chirp chirp chirp on downshift and a single chirp on upshift
and that sounds like a valve not a steady flow out of an airhorn.

the sound on the GTP cars of the era also chirpped out their wastegates and
that is probably what the Audi's also did. I do believe that he is right
that there was not a blowoff valve on the Audi's  (if there was a blowoff
valve it would not have been hooked up to the inlet side of the turbo like
our cars, it would have dumped off right in front of the throttle).  The
wastegate was not muffled on any of those cars so it would have been very

It is called "compressor surge" by Corky Bell, and is even defined in the
Glossary in his book Maximum Boost. Some have said that the Waste gate is
the cause of this Chirp Chirp Chirp, but I don't buy that explanation, as I
don't think you would not be able to hear anything through a regular exhaust
system, open or even with a muffler.

Compressor surge can also occur if you are trying to make a lot of boost at
low RPM's with very little air flow through the exhaust side. (not a good
thing). Most Turbo compressor maps have a "surge" line that runs up the
left side of the map, for low air flow and high pressure ratio events.

I think the main reason you don't hear it on the street cars, is the fact
that they have an air filter and air filter housing designed specifically
to suppress intake noise.

I suppose you could do an experiment on your car, where you removed the one
end of the MAF sensor from the air cleaner box, and let it hang wide open
without the air filter to see if you could hear anything. You could also
then try removing one end of the bypass valve, and plugging the pressure
side hose, to see if the noise was more prominent without the bypass
function during shifting.

But....... you don't want to run up the boost too much and run the risk of
damaging your modified K24 turbo, as the K24 turbos have smaller diameter
shafts, and with the larger cold side impeller, that puts more strain on
the shaft, it might not appreciate this sort of treatment.

There was one 1991 200TQ 20V at Road America last year, which only had a
K&N cone filter on the intake connected to the MAF sensor and it made a lot
more noise when revving up the engine, and then shutting the throttle.

One of the turbo vendors, HKS, or maybe Greddy makes a blow off valve,
which has a small megaphone/horn on it, to make some cool noises during
shifting. I suppose you could rig up the stock bypass valve to operate in a
similar manner to just test out the theory. Unfortunately, the stock valve
is designed to be wide open when the engine is idling, so it would allow
some false air to come in and the car won't idle very well if the bypass
valve intake hose end was left open to the atmosphere, (like a blow off
valve) instead of being connected to the turbo intake as a bypass valve.

by blowing off the pressure during upshifts and deceleration it keeps the
charge air from reversing against the impellers and stalling the turbo. this
could lead to premature turbo failure and slow boost rise after an upshift.

<<the F1 engines of the turbo era used a throttle in front of the turbo
and also in the intake manifold. this had the effect of keeping the boost
pressure captive and ready for when the throttle was opened again.  it had
instant boost after shifts. There wasn't a pressure spike that needed to be
bled off because the turbo suddenly went into vacuum at the inlet side and
couldn't keep packing the inlet track with charge. so for that split second
it just stayed freewheeling and when the throttle blades all opened again it
was at full speed to take a bite of air instantly
I have talked to Turbonetics about that style and he confirmed the
operation of it with me, we are going to give it a try someday (time

Anyone have detailed shots of the bypass/blowoff and intake end of a WRC car
engine bay? Anyone have access to a WRC mechanic who could be even more
definitive about bypass vs. blowoff etc.?